Not applicable.
Not applicable.
The present invention relates to the marking of metal, e.g., for tracking and identification purposes, and more particularly to imprinted anodized aluminum metal tags which can be attached to metal workpieces, e.g., by welding.
A variety of finished goods (e.g., automobile mechanical parts, aerospace parts, etc.) require marking for identification purposes. Such goods may be at or below room temperature when the marking requirement arises. Such goods also may be raw or partly finished and at high temperatures, say, up to 1,100xc2x0 F., in the case of aluminum. Marking of these goods for identification purposes also is required.
In the case of aluminum goods or aluminum tags for attachment to raw, partly finished, and finished goods, the marking of aluminum presents a particularly difficult task because conventional coatings are vulnerable to abrasion. Abrasion resistance of coatings on aluminum relies on the strength of the bond of the coating to the aluminum substrate. Examples of coatings that suffer from good abrasion resistance can be found in U.S. Pat. Nos. 4,873,298 (polysiloxane graft copolymers) and U.S. Pat. No. 3,975,197 (lithographic aluminum plates with a coating of particulate material bound by an aluminum hydroxyoxide coating).
The rapid oxidation of aluminum also creates many problems in obtaining proper adhesion levels of coatings on aluminum substrates. Methods of preparing the surface of aluminum substrates, such as by oxidizing, has been proposed in U.S. Pat. No. 3,664,888. Still, the coated aluminum surface may be compromised even by abrasion testing, for example, with a Taber Abraser. Taber abrasion resistance measures the resistance of a coating applied to a surface, such as metal, to abrasion. The coated surface is subjected to abrasion by rotating the coated panel against weighted abrasive wheels.
In the lithography photographic plate art, there exists an anodized aluminum substrate (aluminum oxide layer formed on the surface of the aluminum by anodic oxidation, J. Elec. Chem. Society, 100, (9), 411), whose surface contains micro-pores. Photosensitive photographic emulsions have been applied to the anodized aluminum plates so that the emulsions become entrapped in the micro-pores (see U.S. Pat. No. 3,615,553). The coated plates then are photographically exposed and wet developed to produce indicia. The aluminum oxide high points are said to surround each exposed micro-pore cavity to protect the exposed indicia. A drawback to such process is the need for photographic exposure to create latent indicia with subsequent wet chemical development to make the indicia visible to the human eye. Tags for on-site marking and identification purposes could not be made practically by such a technique.
One method of preparing a micro-pore aluminum substrate to make it act as a suitable receptor for the impregnating resin is anodizing. Thus, for example, aluminum stock may be anodized in a solution of oxalic acid and oxalates of alkali metals, under controlled pH, current, and temperature, so that the resulting anodized surface is hard, adherent, and is absorbent for soaking up resins and other liquids. Whatever electrolytic solution and anodizing process is used, it generally should be continued for a time sufficient to yield an anodized layer of hard aluminum oxide to a minimum thickness of 0.0002 inches and preferably up to a thickness of 0.05 inches. After the aluminum surface has been initially oxidized, it can then be subjected to one or more powerful oxidizing solutions such as, for example, chromic acid, or solutions of alkali ferricyanides, dichromates, or chromates, which ensure that no metallic aluminum is exposed at the base of the pores prior to them being impregnated with resin. It is critical that this secondary oxidation or xe2x80x9csealingxe2x80x9d step be limited only to the base of the pore and not the entire pore. Over-oxidation seals the entire pore, which results in there being little or no space (volume) for the resin to be absorbed.
After washing and drying this double-oxidized surface, the plate bearing the prepared oxidized aluminum surface then can be impregnated with an alkyl silicone resin. The resulting pore diameter is especially critical, because it must be at least as wide or wider than the smallest particle of resin or other liquid being absorbed. If the pores produced during anodization are too small or the resin does not wet into the cavities, the resin will not penetrate the surface of the aluminum substrate and ultimately, little or no mark indicia will be produced. Polymethyl-type silicone resins are preferred, because once they are properly cured, they produce permanent black markings when subjected to a focused CO2 laser beam. To improve abrasion resistance of the laser marked indicia, excess resin must be removed from the surface of the anodized aluminum before curing, so that the resin, and thus, the marked indicia are only contained within the pores where they are protected by the hard outer layer of aluminum oxide formed during anodization.
The invention, then, is an anodized micro-pore aluminum tag bearing indicia thereon wherein the micro-pore anodized aluminum has its micro-pores filled with the cured resin of a composition, which contains silicone resin having pendant groups selected from one or more of methyl groups or phenyl groups. The composition in the micro-pores was cured to a degree effective for its blackening thereof in the form of a pattern of indicia with a, e.g., CO2, laser beam for marking the tag with indicia as taught in U.S. Pat. No. 5,855,969. The aluminum tag has a surface, which bears the micro-pores, and this surface is substantially free or devoid of said composition, i.e., the composition is present substantially only in the micro-pores.
The method for treating the surface of the anodized micro-pore aluminum tag for forming indicia thereon commences by applying the composition to the surface. Excess of the composition from the surface is removed to leave composition resident in said micro-pores. The composition in the micro-pores then is at least partially cured. A laser then can create the indicia by blackening the composition in the micro-pores.
This removal step of the process desirably includes a first mechanical removal with a blade, i.e., squeegee. Organic solvent for the composition (e.g., ethyl acetate) then can be poured onto the surface and a pool of the solvent squeegeed across the surface to remove the composition from the surface leaving the micro-pores filled with the composition. As a second step, the surface can be rinsed with additional solvent to even out anomalies in the composition in the micro-pores. The composition in the micro-pores then must be cured, at least partially, by heat. A laser can then create the indicia by blackening the composition in the micro-pores.
Advantages of the present invention include the ability to readily laser mark aluminum stock for manufacturing tags without further development. Another advantage includes the ability for form robust indicia recalcitrant to removal by abrasion. Yet another advantage is the ability to form aluminum tags using a rugged CO2 laser. These and other advantages will be readily apparent to those skilled in the art based upon the disclosure set forth herein.